In connection with conventional piston and cylinder type motors having a reciprocable output shaft, mechanisms for locking the shaft against longitudinal displacement are well known in the art. Such mechanisms typically include a locking element which is radially shiftable into an indentation in the shaft, or which, in some cases engages the end of the shaft when the latter is totally extended or retracted.
Fluid driven piston and cylinder combinations of the type provided with means for rotating the shaft as it is longitudinally displaced, thereby resulting in helical displacement of the shaft, are used in many applications where combined rotational and linear movement of a work piece or the like is required. In connection with these latter mentioned motors, there is sometimes also a need for providing lock-up of the shaft against displacement but the design of locking mechanisms for this purpose is considerably complicated by the fact that the shaft possess two degrees of movement, i.e., longitudinal and rotational. One factor complicating the design involves the need to provide precise registration of the reciprocating portion of the locking mechanism and that portion of the mechanism which rotates along with the shaft. In the past, a ramp surface was provided on either the shaft or the associated locking device for the purpose of accommodating manufacturing tolerances ("play") so as to impose a constant axial force on the shaft when the latter is in a locked condition. In the context of a rotatable shaft, however, such ramp surfaces are apt to impart a rotational component of force to the shaft thus tending to release the shaft from its locked position.
Another problem associated with prior art locking mechanisms for helically displaceable shafts involves the fact that the movable portion of the mechanism, usually the locking element which slides into an indentation of the shaft or associated components, requires a substantial amount of travel due to the fact that the locking element shifts in a radial direction toward or away from the longitudinal axis of the shaft.
It may therefore be appreciated that the prior art locking mechanisms described above are less than completely satisfactory in terms of their complexity and reliability. The present invention is directed toward overcoming each of the difficiencies discussed above.